A typical high frequency amplifier circuit operable in the frequency bands mentioned above has a circuit configuration comprising a bipolar semiconductor (hereinafter referred to as semiconductor) with its emitter grounded, its base connected with a signal line, and its collector connected with a load or impedance for generating an output signal.
In order no enhance its output power (i.e. electric power be supplied to the impedance connected to the output terminal of the amplifier circuit), a low-impedance emitter follower is often provided in the output stage of the circuit, yielding its output from the output terminal of the emitter follower, as shown in FIG. 1.
Such an amplifier circuit as illustrated in FIG. 1 includes an emitter-grounded amplifying stage comprising an NPN transistor Q1 whose emitter and base are connected with a grounded terminal 1 and a bias resistor RB, respectively, and a load resistor R.sub.L connected between the collector of the transistor Q1 and a high-voltage power source 2. An emitter follower of the output stage includes another NPN transistor Q2 having a collector connected with the high-voltage power source 2 and a base connected with the collector of the transistor Q1 of the amplifying stage, and a resistor R.sub.C connected between the emitter of the transistor Q2 and the grounded terminal 1. In this amplifier circuit a signal input to the input terminal 3 is amplified by the amplifying stage formed of the transistor Q1 and the load resistor R.sub.L, and amplified signal is output from the output terminal 4 via the emitter follower of the output stage formed of the transistor Q2 and a resistor R.sub.C.
Since such amplifier circuit has a low output impedance associated with the emitter follower serving as an output stage, the output driving power is improved greatly, thereby saving power consumption in the circuit.
Recently, there exists a further need of improvement in amplifier circuits for use with battery operated electronic circuits, such as portable wireless telephone systems, operable in high frequency bands. It is then necessary to further reduce power consumption in such electronic circuits.
However, such an amplifier circuit as shown in FIG. 1 has a disadvantage that, if its operational current is lowered to reduce energy consumption, output drive power available for a load is diminished and the output signal is distorted, though its output impedance is lowered by the emitter followers.
There have been known amplifier circuits, as shown in FIGS. 2 and 3, which may provide a fairly large output driving power while suppressing the energy consumption.
FIG. 2 is a circuit reported in NEC Technical Report, vol. 41, No. 14, PP. 260-263 (1988), which has a higher output driving power than the foregoing amplifier circuit. (It should be understood that components which are not relevant to basic amplifying operations are not shown in FIG. 2 for brevity.) This amplifier circuit has a resistor R1 connected between the emitter of a transistor Q1 in an amplifying stage and a grounded terminal 1, thereby forming an emitter follower. The signal retrieved from the emitter follower is then applied to the base of a transistor Q3 serving as a constant current source, so that a large output driving power may be obtained. Connected between the base of the transistor Q1 and an output terminal 4 is a resistor R.sub.B for providing a base bias voltage under self-biasing scheme. Also, connected between the emitter of the transistor Q3 and the grounded terminal 1 is a resistor R2 for determining the current through the transistor Q3.
In this amplifier circuit signals input to a terminal 3 is retrieved from the output terminal 4 through the transistors Q1 and Q2 on one hand, and, on the other hand, through the emitter follower formed of the transistor Q1 and the resistor R1 and via the transistor Q3. In order for the transistor Q1 to act as an emitter follower, it is necessary that the resistor R1 is fairly great. However, if R1 is great, high frequency characteristics of the amplifying stage consisting of the transistor Q1 and a load resistor R.sub.L are undesirably deteriorated in its fidelity in comparison to the emitter-grounded amplifier circuit shown in FIG. 1. In fact, as described in the above literature, the circuit shown in FIG. 2 is designed as an IF amplifier circuit for use in an output stage of a down converter for 100 MHz or below, with its nominal operational frequency at 1 GHz or below, so that it would not be suitable for amplification in frequency bands above 1 GHz.
The second example shown in FIG. 3 has a portion functioning as an emitter follower as of transistor Q1 of FIG. 2 and another portion functioning as a signal transfer circuitry, as of the transistor Q1 of FIG. 2, for transferring signals to a transistor Q3. Separation of the two functions results in improved high frequency characteristics when compared to the amplifier circuit shown in FIG. 2. (For brevity of representation elements irrelevant to basic amplifying operations are also abbreviated in FIG. 3.) This circuit is discussed in IEEE Journal of Solid-State Circuits, Vol. 24, No. 1, 7-12 (February, 1989). In this amplifier circuit, the base of the transistor Q1 is self-biased by means of a bias circuitry formed of a resistor R.sub.B connected with the base of the transistor Q1 in a amplification stage and of a diode D connected with a constant current power source C and the emitter of the transistor Q1. The diode D is included in this manner so that the output of an emitter follower consisting of a transistor Q4 and a resistor R3 may be coupled directly to the base of a transistor Q3. The transistors Q2 and Q3 constitute an output emitter follower of the output stage of the circuit. Thus, the transistor Q3 is provided on its base with the signal having the same phase as the input signal.
Since the amplifier circuit shown in FIG. 3 has an amplifying stage (consisting of the transistor Q1 and a load resistor R.sub.L) separated from the emitter follower (consisting of the transistor Q4 and the resistor R3) for applying signals to the base of the transistor Q3 serving for driving the emitter follower of the output stage, the circuit has better high frequency characteristics than the one shown in FIG. 2. However, the high frequency characteristics of the circuit are inferior to those of the emitter-grounded circuit shown in FIG. 1. This is due to the fact that the diode D is connected between the emitter of the transistor Q1 of the amplifying stage and the grounded terminal 1 for equalizing the potential levels of the emitter and the terminal 1. Further, although the resistor R.sub.B provides self-biassing to the transistor Q1 in controlling the current through it, it is the resistor R2 that provide feedback to the transistor Q3 to control the current through the transistor Q3. Thus, as the current I.sub.Q3 through the transistor Q3 is increased by reducing the resistance of the resistor R2, energy consumption increases, while the output driving power is decreased as the current I.sub.Q3 is decreased by increasing the resistance of the resistor R2. In fact, as stated in the above literature, energy consumption in the circuit significantly increases as much as 540 mW under 12 Volts if its output driving power is to be maintained at a nominal level and output waveforms be maintained not distorted. Such energy consumption is not acceptable from the point of energy saving policy.
In short, when operated for nominal output driving power, prior art amplifier circuits disadvantageously have large energy consumption, much too complicated structures, or poor high frequency characteristics.